Material Machining Device And Method

ABSTRACT

The aim of the invention is to machine a material by application of non-linear radiation. The aim is achieved by modifying the laser radiation emitted by a laser beam source with the aid of a polarization modulator in such a way that laser radiation focused into the material is polarized in a linear fashion, the direction of polarization varying across the cross section of the beam.

The invention relates to a machining device which machines a material bynon-linear absorption of machining laser radiation and which comprises alaser radiation source emitting laser radiation, and optics focusingsaid laser radiation for non-linear absorption into or onto thematerial. The invention further relates to a method of machining,wherein the laser radiation is focused into or onto the material, theparameters of the laser radiation and of focusing being selected suchthat non-linear absorption of the laser radiation is caused in thematerial.

Laser-based machining is indispensable today in many areas of industrialproduction. The machining quality achieved by the use of lasers can notbe obtained in many cases by alternative methods. On top of this, highprocessing speeds are realized in automated production, allowingeconomically advantageous application of laser-based methods. This canbe effected by a multiplicity of complex devices and systems, which areadapted as workstations for a number of specific machining tasks.Industrial workstations comprise, for example, systems for cutting metalsheets as well as devices for machining structures on a microchip. Thereare also laser systems for carrying out medical processes, for examplefor correction of eyesight in the case of myopia or hyperopia. Theselaser systems effect machining of human tissue, while in industrialapplications solid materials, such as steel or ceramics, are machined inmost cases. Applications for liquid materials, for examplelaser-assisted polymerization in rapid prototyping, are also important.

If an object is irradiated with laser radiation, this is effected inmost cases in order to effect a measurement or in order to machine thematerial of the object. In each case, there is an interaction betweenthe laser radiation and the material, for example a reflection, ascattering process or an absorption. Important applications in thiscontext are interferometric measuring methods, conventional lasermicroscopy or exposure of photolacquers in semi-conductor lithography.

However, there are also applications in which conventional, i.e. linearinteraction, between the laser radiation and the material is notpossible, for example because the material is transparent. In suchapplications, non-linear interaction between the laser radiation and thematerial can be utilized for machining, i.e. in particular non-linearabsorption of the laser radiation by the material of the object. In thisconnection, reference is made to n^(th) order absorption, if anabsorption of m photons is effected by an atom or a molecule, leading toan n-fold electronic excitation. It has turned out in this case that thelikelihood of such n^(th) order absorption depends on the radiationintensity of the laser radiation.

In a transparent material, as it is present, in particular, inlaser-surgical ophthalmic methods, several processes initiated by thelaser radiation take place one after another in the case of non-linearinteraction. If the power density of the radiation exceeds a thresholdvalue, an optical breakthrough is produced in the transparent material,said breakthrough generating a plasma bubble in the material. Due toexpending gases, this plasma bubble grows after the optical breakthroughhas formed. If the optical breakthrough is not maintained, for examplebecause pulsed laser radiation has been used, the gas generated in theplasma bubble is re-absorbed by the surrounding material and the bubbledisappears. If a plasma is generated at a material boundary surface(which may actually be located within a material structure as well),material removal is effected from said boundary surface. This is thenreferred to as photoablation, whereas in connection with a plasma bubbleseparating previously connected material layers one usually speaks ofphotodisruption. For the sake of simplicity, all such processes aresummarized here by the term “optical breakthrough”, i.e. this termincludes not only the actual optical breakthrough itself, but also theeffects resulting therefrom in the material, caused by the non-linearinteraction.

High localization of the effect of the laser radiation is required forhigh-precision machining. Collateral damage in adjacent regions of thematerial is to be avoided, if possible.

Looking at the probability of non-linear absorption, i.e. multi-photonabsorption, it turns out that the probability of such interaction in alaser focus generated by a diffraction-limited optical system isproportional to the n^(th) power of the quotient of laser power timesthe square of the numerical aperture and the square of the wave lengthused. For machining at maximum speed by means of non-linear absorptionit is desired, of course, to optimize the likelihood of interactions. Anincrease in such likelihood of multi-photon absorption can be achievedby increasing the laser power, increasing the numerical aperture ordecreasing the wave length of use of the laser radiation, as is directlyevident from the above-explained context. Therefore, it has beenindispensable hitherto in the prior art of non-linearly effective lasermachining concepts to use optical systems having a high numericalaperture for focusing.

U.S. Pat. No. 5,894,916 as well as U.S. Pat. No. 6,110,166 describemethods for correcting visual deficiencies by means of suitablygenerating optical breakthroughs in the human cornea, so that thediffractive properties of the cornea are ultimately influenced in aselective manner. By sequential arrangement of the multiplicity ofoptical breakthroughs, a lens-shaped partial volume is isolated withinthe cornea of the eye, said volume then being removed from the cornea bymeans of a laterally opening cut. The shape of the partial volume isselected such that the diffractive properties of the cornea are changedafter removal of the partial volume to achieve a desired correction of avisual deficiency.

It is an object of the invention to improve a device for machining bymeans of laser radiation of the above-mentioned type or a correspondingmethod of machining, respectively, such that the generation ofnon-linear interactions is improved.

According to the invention, this object is achieved by a machiningdevice of the above-mentioned type, said device comprising apolarization modulator which causes the focused laser radiation to belinearly polarized, with the direction of polarization varying acrossthe beam cross-section. In an analogous manner, this object is achievedby a method of the above-mentioned type, wherein laser radiation islinearly polarized before focusing, with the direction of polarizationvarying across the beam-cross section.

Thus, according to the invention, an inhomogeneously polarized field ofradiation is used. Although the laser radiation is linearly polarized ineach point of the field, it may have a different direction ofpolarization in each point—in contrast to usual distributions.Surprisingly, the inhomogeneously polarized field of radiation accordingto the invention leads to a reduction in focus size, thereby increasingthe power density in the focus of the laser radiation and thus also thelikelihood of initiating multi-photon processes. Previously, said focusreduction had only been assumed theoretically (Quabis et al., Opt. Com.,179, 2000) or discussed experimentally for optical data storage,respectively (Dorn et al., J. Mod. Opt. 50, 12, p. 1917-1926, 2003).Surprisingly, the concept of continuous wave radiation (cw radiation)discussed throughout the prior art for linear interaction can also beadvantageously used for non-linear interactions, and in particular whenusing pulsed laser radiation. For example, in the case of theabove-described machining by means of optical breakthroughs the energytransmitted into the material can now be minimized, thus keepingcollateral damage low. If several optical breakthroughs are sequentiallyarranged by the concept according to the invention in order to effectareal material separations, an improved quality of cutting is achieveddue to the reduced focus obtained according to the invention.

The object is further achieved by a machining device of theabove-mentioned type, which device comprises an intensity modulatormodifying an intensity distribution of the laser radiation whileattenuating the radiation intensity near the optical axis. In ananalogous manner, the object is achieved by a method of theabove-mentioned type, wherein the intensity distribution of the laserradiation is modified prior to focusing, while the radiation intensitynear the optical axis is being attenuated.

Thus, the polarization modulator and the intensity modulator can be usedalternatively in order to achieve the object of the invention. Theintensity modulator causes a suitable change in the beam profile. Theinventors have found that a beam profile modification reducing theradiation intensity near the optical axis surprisingly also leads to areduced focus size, i.e. to better focusing. Since conventional sourcesof laser radiation usually emit a laser beam having a Gauss-shapedprofile, the intensity modulator for such machining devices has theeffect that regions viewed along the beam profile, located far from theoptical axis, bear a proportionally higher percentage of intensity thanin the Gauss-shaped profile.

In a simple design of the intensity modulator, it blocks out theseradiation components near the optical axis or attenuates them.Possibilities to realize this exist, for example, in the shape of apoint-shaped stop or a punctiform absorbing unit located at the centerof the beam-cross section, or of a reflector deflecting the beam path,which reflector, located at the center, reflects poorly or not at all.Any power losses connected therewith throughout the cross-section of thelaser radiation can be compensated for by suitably increasing theradiation power preceding the intensity modulator.

If it is desired to leave the radiation power of the laser radiationunchanged, if possible, an intensity modulator is suitable which effectsa re-distribution of energy in the beam cross-section, wherein energy isdistributed away from regions located near the optical axes toward outerregions of the beam cross-section. An example of the optical principleswhich an intensity modulator acting in this way has to comply with isfound, for example in U.S. Pat. No. 4,826,299, the disclosure of which,in particular with respect to the optical parameters of FIG. 16 therein,is explicitly incorporated herein by reference.

The use of non-Gauss-shaped laser radiation and in particular laserradiation having low intensity at the center of the laser beam is quitea surprising result considering the fact that the optical quality ofobjectives used for focusing is usually much better near the opticalaxis than in regions remote from the axis. It is a well-established factin the prior art that, in order to increase the quality of the focus,the radiation should be guided, if possible, near the optical axis,because radiation guided in regions remote from the axis usually leadsto an astigmatism of the image.

The above-mentioned intensity modulation can be used as an alternativeor in addition to the aforementioned polarization modulation. Thus,where general reference is made hereinafter to modulation or to amodulator, respectively, this may mean both intensity modulation andpolarization modulation. It is also possible to connect effects ofpolarization and of radiation intensity via the beam-cross section in amutually adapted manner.

A further surprising advantage of the invention shows in the inventiveprocedure in combination with multi-dimensional shifting of the focusposition, because the excellent quality of focus achieved by theinvention otherwise required very large numerical apertures in the priorart, which apertures were a hindrance to a large area of deflection forthe focus but can now be smaller.

The inventors realized that using the approach according to theinvention in presently already realized optical systems for machining inconnection with refractive eye surgery by means of femtosecond laserradiation, a reduction of the focus diameter by up to 30% is achieved.This reduces the radiation exposure by up to 50% when generating asingle optical breakthrough, or if the radiation load associated withmachining is kept constant, a duplication of the repetition frequency ofthe pulsed laser radiation and thus a fifty percent reduction of thetotal machining time can be achieved.

Thus, according to the invention an increase in energy or power density,respectively, in the laser focus for machining by laser is realized byproviding

-   a laser radiation source,-   the optical module referred to as a modulator,-   preferably a deflecting unit which controls the spatial position of    the focus of the laser radiation with respect to the material by    influencing the direction and/or divergence of the laser beam, and-   an objective which is designed such that it is adapted for focusing    the laser radiation into the material in order to achieve efficient    non-linear absorption of the laser radiation in the material.

The laser radiation interacting non-linearly with the material ispreferably pulsed and has a high peak power, as is the case for examplewith femtosecond laser radiation. The modulator, which is a polarizationmodulator in one embodiment, uses one or several structural componentsfor converting homogeneously linearly polarized laser radiation, such asthat which is usually emitted by laser radiation sources, toinhomogeneously linearly polarized laser light.

In order to produce a cut surface, multi-dimensional shifting of thefocus in or on the material is conveniently effected, as alreadymentioned. Therefore, a further embodiment is preferred which comprisesa deflecting unit modifying a spatial position of the focus in or on thematerial by controllable deflection of the laser radiation.

What is essential in achieving the superior focus quality according tothe invention is the polarization direction varying across the beamcross-section of the linearly polarized laser radiation and the reducedradiation intensity near the optical axis. Therefore, for a particularlyeasy to realize construction, the modulator will be arranged followingthe laser radiation source and, in the case of further embodimentscomprising a deflecting unit, will be arranged between the laserradiation source and the deflecting unit. The modulator isadvantageously located in any pupil plane of the beam path between thelaser radiation source and the material to be machined. The position ofthe intensity modulator is not fixed and can be adapted to anapplication by means of a propagation calculator. In a special casewhich is particularly easy to understand, the modulation plane and thefocal plane are conjugated; the beam profile in the focus thencorresponds directly to the Fourier-transformed beam profile followingthe modulator.

As an alternative to the modulator subsequently modulating homogeneouslylinearly polarized or Gauss-shaped laser radiation, the polarizationmodulator or the intensity modulator can also be arranged directly inthe laser radiation source, so that the latter already emits laserradiation having a polarization direction which varies over the beamcross-section or having a non-Gauss-shaped intensity distribution.

It has been shown that, according to the geometry of the beam path bywhich the laser radiation is focused into or onto the material anddepending on the material, different forms of inhomogeneously linearlypolarized laser radiation or laser radiation differing in the intensitydistribution within the profile may be preferred. In terms of universalusability of the machining device or applicability of the machiningmethod it is convenient if the modulator is adjustable or in particularcontrollable so as to vary the polarization direction or to change theintensity distribution. Adjustment can be conveniently effected duringoperation of the device, for example by intervention of a correspondingcontrol unit.

Using a controllable modulator, quality control of machining canadvantageously be effected such that a measured value determined duringmachining, for example a quality parameter, is suitably controlled usingthe variation of the polarization directions or the change in intensitydistribution as manipulated variables.

The concept according to the invention is surprisingly advantageouswhere pulsed laser radiation having a very short pulse duration is used,thus making more efficient use of the energy introduced into thematerial for machining. It is therefore preferred that the laserradiation source emit pulsed laser radiation having a pulse duration ofless than 10,000 fs, in particular less than 500 fs. A particularlyoptimal value in the field of ophthalmic surgery has turned out to beapproximately 350 fs. If the pulses are even shorter, the increasingspectral width of the pulses increases the technical complexity.

The inhomogeneously linearly polarized laser radiation used according tothe invention further enables rapid machining; for example, in the caseof pulsed laser radiation, a pulse repetition frequency of over 100 kHz,in particular of over 450 kHz, is possible.

For non-linear interaction of the laser radiation with the material,focusing may be effected, on the one hand, into the material, i.e. belowits surface. However, it is also possible to focus the laser radiationin the vicinity of the surface of the material to be machined. Aparticularly good photoablative effect is achieved if the distance ofthe laser focus from the surface of the material to be machined isapproximately within the range of the Rayleigh length of laserradiation.

The laser radiation to be used according to the invention isparticularly convenient if, in particular by areal sequentialarrangement of optical breakthroughs in the material, cut surfaces areformed which are located in the material and may have a cutting lineextending up to the surface of the material.

The invention will be explained in more detail below, by way of exampleand with reference to the drawings, wherein:

FIG. 1 shows a perspective view of a patient during laser-surgicaltreatment using a laser-surgical instrument;

FIG. 2 shows a schematic view of the focusing of a ray bundle onto thepatient's eye with the instrument of FIG. 1;

FIG. 3 shows a schematic view illustrating the cut path effected duringlaser-surgical treatment;

FIG. 4 shows a schematic view of a first embodiment of a laser machiningdevice for generating wave guide structures;

FIG. 5 shows an example of inhomogeneously linear polarization of alaser beam according to the first embodiment comprising sectorialpolarization variation;

FIG. 6 shows a further example of inhomogeneously linear polarization ofa laser beam according to the first embodiment with radial polarizationvariation;

FIG. 7 shows a further example of inhomogeneously linear polarization ofa laser beam according to the first embodiment with discontinuous radialpolarization variation;

FIG. 8 shows a top view of a polarization modulator of the firstembodiment for generating processing laser radiation with inhomogenouslylinear polarization;

FIG. 9 shows the polarization directions of the laser radiation uponapplication of the polarization modulator of FIG. 8;

FIG. 10 shows a schematic view of a second embodiment of a lasermachining device similar to FIG. 4;

FIGS. 11 and 12 show different variants of a structural component whichmay be used in the laser machining device of FIG. 10;

FIG. 13 shows beam profiles which are possible in the laser machiningdevice of FIG. 10;

FIG. 14 is a schematic drawing illustrating a further variant of thestructural element of FIGS. 11 and 12;

FIG. 15 is a sectional view of the structural element of FIG. 14, and

FIG. 16 is a beam profile obtainable with the structural component ofFIG. 15.

FIG. 1 shows a laser-surgical instrument 1 emitting a treatment beam 2which is directed onto the eye 6 of a patient. The laser-surgicalinstrument 1 generates a pulsed treatment beam 2 such that the methoddescribed in U.S. Pat. No. 6,110,166 can be carried out. The pulsedduration is in the nano- or femtosecond range.

Using the laser-surgical instrument 1 a visual deficiency of the eye 6of the patient is remedied by removing material from the cornea suchthat the diffractive characteristic of the cornea is changed to adesired extent. In doing so, material is removed from the stroma of thecornea, said stroma being located below the epithelium and the Bowmanmembrane as well as above the Decemet membrane and the endothelium.

The laser-surgical instrument 1 machines a material which is actuallytransparent to laser radiation. The pulsed laser radiation of thetreatment beam 2 is therefore irradiated such that non-linear absorptionoccurs in order to generate the optical breakthrough.

Material removal is achieved by focusing the high-energy pulsedtreatment beam 2 of the laser-surgical instrument 1 into the stroma,thereby generating optical breakthroughs. Each single opticalbreakthrough initiates a plasma bubble so that tissue is separated. Bysuitable deflection of the treatment beam 2, plasma bubbles aresequentially arranged during treatment, which bubbles then circumscribea partial volume of the stroma, namely the material to be removed. Dueto the treatment beam 2, the laser-surgical instrument 1 acts like asurgical knife which, without injuring the surface of the cornea, cutsmaterial directly within the transparent cornea of the eye. If the cutis guided up to the surface of the cornea by further generating opticalbreakthroughs, the material of the stroma isolated by the cut profilecan be pulled out laterally and removed from the cornea.

As shown in FIG. 2, the laser-surgical instrument 2 comprises a lasersource 9 whose treatment beam 2 is focused as focused laser radiation 8into the cornea 4 of the eye 6 by means of an (adjustable) objective 3.The already mentioned plasma bubble 11 is then formed in the focus.

FIG. 3 schematically shows how a cut surface is generated in the cornea4 in order to isolate a partial volume 5. By suitable three-dimensionalshifting of the focused laser beam 8, many plasma bubbles 11 aresequentially arranged in order to form a cut surface (shown in brokenlines in FIG. 3). If the partial volume 5 is isolated within the cornea8, the cut is guided up to the periphery such that material of thepartial volume 5 can be pulled out in the direction of the arrow 7.

The structure of a laser treatment instrument is shown in simplifiedform in FIG. 4 for a first embodiment, said instrument beingsubstantially similar to the laser-surgical instrument 1, but in thiscase serving to form a wave guide structure 14 in a transparent material13. A polarization modulator 10 (which, although not mentioned, is alsoprovided in the beam path of the laser-surgical instrument 1) isarranged following the laser source 9, said polarization modulatormodifying the linearly polarized radiation 2 emitted by the laser source9 such that it has inhomogeneous linear polarization conditions.

The inhomogeneous linear polarization conditions are characterized inthat the polarization vector {right arrow over (P)} of the field doesnot have a uniform direction across the cross-section of the collimatedlaser beam.

For physical characterization of such polarization conditions it shouldbe noted first that for a collimated laser beam the field strength canbe approximated conventionally as a planar wave. The position of thefield strength vector can then be represented for linearly polarizedlight in any x, y coordinate system in which the wave propagates in thedirection of z:{right arrow over (E)}={E _(0x) , E _(0y)}exp i( ωt−kz)

For simplification, the standardized polarization vector {right arrowover (P)} of the field can now be introduced, the amplitude of the fieldstrength being determined completely by the amplitude function √{squareroot over (E_(0x) ²+E_(0y) ²)}. Although said amplitude function dropsrapidly for a Gauss beam, the following equation applies to thepolarization vector, regardless of the specific amplitude distribution:$\overset{arrow}{P} \equiv \frac{{E_{0x}{\overset{arrow}{e}}_{x}} + {E_{0y}{\overset{arrow}{e}}_{y}}}{\sqrt{E_{0x}^{2} + E_{0y}^{2}}}$Thus, the field strength results as:{right arrow over (E)}=√{square root over (E _(0x) ² +E _(0y) ²)}·{rightarrow over (P)}·exp i(wt−kz)

The polarization vector is a function of x and y. If{right arrow over (P)}={right arrow over (P)}(x, y)=const.then this is a homogeneously linearly polarized field such as that whichis emitted by most lasers, for example by a helium-neon laser. If{right arrow over (P)}={right arrow over (P)}(x, y)≠const.then the corresponding field of radiation is inhomogeneously linearlypolarized.

After an inhomogeneously linear polarization has thus been imposed onthe laser radiation 2, the latter passes through a deflecting unit 12,which is symbolized here in the form of two deflecting mirrors beingindependently tiltable about orthogonal axes. Together with theadjustable objective 3, the focused laser beam 8 can thus bethree-dimensionally shifted within the volume of the transparentmaterial 13, thus allowing to form three-dimensional structures in thematerial, for example the wave guide structure 14 as shown. The pulsedlaser radiation 2 emitted by the radiation source 9 is non-linearlyabsorbed by the material 13, thus causing a permanent change in materialproperties within the spatial region machined. As an alternative or inaddition to the effect of the deflecting unit 12 and the adjustableobjective 13, it is certainly also possible to move the object with thetransparent material 13 itself relative to the objective 3. This may beof interest in particular if structures are to be generated whose extentgoes at least partially beyond the positions achievable by thedeflecting unit 12 or the length of adjustment of the objective 3.

Due to the inhomogeneously linear polarization of the focused laser beam8, a focus is obtained in the transparent material 13, which focus hassmaller dimensions than would actually expected due to the beam path andin particular due to the objective 3. The precision of machining is thusincreased. The same goes for the instrument 1 of FIG. 1.

The focus reduction achieved by the polarization modulator 10 may differaccording to the beam path and in particular according to the type oftransparent material 13. For optimization, it is therefore convenientlyenvisaged that the quality of machining, for example the size of thespatial region in which a non-linear interaction is determined, ismeasured and an optimization is effected by varying the setting of thepolarization modulator 10. For example, the size of a plasma bubblegenerated in the material 13 or in the cornea 4 can be measured andminimization can be effected by adjusting the polarization modulator 10which is controlled by a control device (not shown). Alternatively, theradiation emitted by the laser source 9 can also be gradually reduced bysuitable control of the polarization modulator 10 until a minimalradiation power or pulse duration is present which is still sufficientfor the desired non-linear interaction.

The effect of the polarization modulator 10 is shown by way of examplein FIG. 5, showing an x-y section 15 through the laser beam 2 whichpropagates in the direction of z (perpendicular to the drawing plane).As can be seen, the polarization modulator 10 imparts to the laser beam2 an inhomogeneous polarization distribution in such a form that twopartial regions 16 and 17 are present in which the polarizationdirection, although being respectively identical, has a phase shift of180° with respect to the other partial region. This is symbolized inFIG. 5 by the arrows which are parallel, but point in oppositedirections. Instead of an inhomogeneity comprising two partial regions16 and 17, it is also possible, of course, to use any other sectorialpolarization.

FIG. 6 shows a radial polarization distribution in the x-y section 15.The polarization directions point radially outwardly from the opticalaxis A.

FIG. 7 shows an inhomogeneous polarization comprising phase jumps withinannular portions in the x-y section 15. The phase jumps appear at jumplocations 18 concentrically arranged about the optical axis A. The laserbeam 2 is divided into four rings 19 having different polarization, asviewed in the x-y section 15. Between adjacent rings there is a phasejump of the polarization direction, which jump is shown as an 180° phasejump for reasons of illustration. Of course, any other phase jump isalso possible.

In order to form one of the inhomogeneous linear polarization conditionsof FIGS. 5 to 7 the polarization modulator 10 according to a preferredalternative embodiment comprises a segmented wave plate which causes thedesired polarization modulation alone or in combination with otheroptical components. In a further alternative embodiment, dichroic glasspolarizations are provided which are designed according to the desiredpolarization distribution for the respective application. Also, liquidcrystal components can be used, which are usually referred to as spatiallight modulators (SLM) and enable flexible adaptation of thepolarization distribution to a respective processing task, controlled bya corresponding adjusting signal.

FIG. 8 shows a further possible design of a polarization modulator 10 asknown for a completely different application from EP 0,764,858 A2, whosedisclosure is fully incorporated herein by reference. It refers to ahoneycomb condenser 20 comprising a multiplicity of honeycomb-shapedcells 21. The cross-section of the laser beam 2 is thus divided into amultiplicity of facets 21. Each facet 21 consist of a half-wave plate ofbirefringent material and is oriented, for example, according to thepattern shown in FIG. 8. Therefore, they are each oriented in thedirection of the angle bisector between the polarization direction P,also shown in FIG. 8, of the incident linearly polarized laser radiation2 and the respective radius, oriented towards the optical axis A,through the center of each facets 21. Thus, each half-wave plate facet21 effects rotation of the polarization direction in the direction ofthe aforementioned radius.

FIG. 9 shows the effect of this arrangement of the main axes 22 of thefacets 21 of the honeycomb condenser 20, due to the polarizationdirections 23 being radially aligned with the optical axis A afterpassage.

FIG. 10 is a simplified view of a second embodiment of a design for alaser processing instrument 1 which is essentially similar to that ofFIG. 4. The difference in design concerns the modulator arrangedfollowing the laser source 9, which modulator is provided as anintensity modulator 24 here. The remaining elements correspond to thosealready explained for the first embodiment, so that their description isnot repeated here.

The intensity modulator 24 modifies the beam profile of the treatmentbeam 2 such that the radiation intensity near the optical axis isattenuated. Therefore, the intensity modulator 24 is located in a planeconjugated with the focal plane in the construction shown in FIG. 10.The intensity modulator 24 employs a modulating element 25 for which twopossible constructions are shown in FIGS. 11 and 12.

The element 25 in the construction of FIG. 11 is formed from atransparent substrate 26 onto which a circular stop element 27 isapplied here. The stop element 27 has a diameter D, which covers, forexample, 50% of the beam cross-section of the treatment beam 2. Insteadof a covering stop element 27 use can also be made of an absorbingelement 27 attenuating, but not shading, the treatment laser beam 2 inthe core region indicated by hatching.

The same result is obtained with the construction of the element 25shown in FIG. 12; however, this variant 27 works reflectively. For thispurpose, a mirror coating 28 is provided, which is transparent orpartially transparent in a circular region 29 within the beamcross-section.

Those radiation components passing through the region 29 are separatedfrom the treatment beam 2 and only the reflected components are bundledin the focus. Instead of being fully transparent, the circular region 29may also be partially reflective, which again results in attenuation inthe core region of the treatment laser beam 2.

An exemplary result for the beam profile obtainable with the element 25is shown in FIG. 13. The initially Gauss-shaped profile 30, plotted asthe intensity I against the location x in the left-hand graph of FIG.13, is deformed by the effect of the intensity modulator 24 to a profile31 which is considerably attenuated at the center with a diameter B. Itshould be noted that the illustrations of the left-hand graph and theright-hand graph of FIG. 13 do not have the same scale on the intensityaxis; the right-hand graph is enlarged in terms of intensity so as tomake the effect clear.

As a result, the treatment beam 2 is no longer Gauss-shaped in profileafter the intensity modulator 24, which likewise leads to an improvedquality of focus as does the polarization modulation already explainedwith respect to the first embodiment.

A further possibility of realizing the intensity modulator 24 is shownin FIGS. 14-16. In this case, a Powell asphere 33 is provided on asubstrate 32 in the intensity modulator 24, which asphere spreads thetreatment beam 2 with a Gauss-shaped profile out in a certain mannersuch that a diverging ray bundle 34 forms. As the representation ofexemplary partial beams in FIG. 14 clearly shows, the Powell asphere 33causes an energy re-distribution away from the optical axis. Following aconverging lens (not shown in FIG. 14 for simplification) and/or adiffractive optical element, the treatment beam 2 is then present with anon-Gauss-shaped profile. An example of the achievable profile is shownin FIG. 16. As can be seen, the intensity I of the modified treatmentbeam 2 is distributed in a near-rectangular profile 37 across the beamcross-section.

The effect of the Powell asphere 33 is primarily due to its cone shape,as viewed in cross-section. FIG. 15 shows this cone 35. The point of thecone 35 is rounded off at a certain radius 36. Details concerning theconstruction and design of the Powell asphere 33 are found in thealready mentioned U.S. Pat. No. 4,826,299.

In a further advanced embodiment, the instrument 1 may use both apolarization modulator 10 according to one of the describedconstructions and an intensity modulator 24 in one of the describedconstructions. Conveniently, the polarization modulator may also realizean amplitude modulation at the same time. For this purpose, the rings 19of FIG. 7 or the cells of facets 21 of FIG. 8 may be provided, forexample, with suitable radiation intensity-influencing properties, sothat the radiation intensity at the center of the beam is attenuated. Acombination of the polarization modulator 10 with the intensitymodulator 24 in one component leads to a particularly compact design. Acombination of a separate polarization modulator 10 with an individualintensity modulator 24, on the other hand, can be realized atcomparatively low cost.

1-19. (canceled)
 20. A machining device which machines a material bynon-linear absorption of machining laser radiation, comprising: a laserradiation source emitting said laser radiation; optics focusing thelaser radiation for non-linear absorption into or onto the material; anda polarization modulator which causes the focused laser radiation to belinearly polarized, with a polarization direction varying across thebeam cross-section.
 21. The machining device as claimed in claim 20,further comprising a deflecting unit which modifies a spatial positionof the focus in the material by controllable deflection of the laserbeam.
 22. The machining device as claimed in claim 20, wherein themodulator is arranged between the laser radiation source and thedeflecting unit.
 23. The machining device as claimed in claim 20,wherein the laser radiation source emits linearly polarized radiationand the polarization modulator inhomogeneously modifies the polarizationdirection of the laser beam across the beam-cross section.
 24. Themachining device as claimed in claim 20, wherein the modulator isarranged within the laser radiation source such that the laser radiationsource emits laser radiation having a polarization direction whichvaries across the beam cross-section or with modified intensitydistribution.
 25. The machining device as claimed in claim 20, whereinthe polarization modulator is adjustable, with respect to the variationof the polarization direction.
 26. The machining device as claimed inclaim 25, further comprising a control unit which modifies the variationof the polarization direction during operation of the machining device.27. The machining device as claimed in claim 20, wherein the laserradiation source emits pulsed laser radiation with a pulse duration ofless than 10,000 fs.
 28. The machining device as claimed in claim 27,wherein the laser radiation source emits pulsed laser radiation with apulse duration of less than 500 fs.
 29. The machining device as claimedin claim 25, wherein the laser radiation source operates at a pulserepetition frequency of more than 100 kHz.
 30. The machining device asclaimed in claim 25, wherein the laser radiation source operates at apulse repetition frequency of more than, 450 kHz.
 31. A machining devicewhich machines a material by non-linear absorption of machining laserradiation comprising: a laser radiation source emitting said laserradiation, optics focusing the laser radiation for non-linear absorptioninto or onto the material; and an intensity modulator which modifies anintensity distribution of the laser radiation, thereby attenuating theradiation intensity near the optical axis.
 32. The machining device asclaimed in claim 31, further comprising a deflecting unit which modifiesa spatial position of the focus in the material by controllabledeflection of the laser beam.
 33. The machining device as claimed inclaim 31, wherein the modulator is arranged between the laser radiationsource and the deflecting unit.
 34. The machining device as claimed inclaim 31, wherein the modulator is arranged within the laser radiationsource such that the laser radiation source emits laser radiation havinga polarization direction which varies across the beam cross-section orwith modified intensity distribution.
 35. The machining device asclaimed in claim 31, wherein the laser radiation source emits pulsedlaser radiation with a pulse duration of less than 10,000 fs.
 36. Themachining device as claimed in claim 35, wherein the laser radiationsource emits pulsed laser radiation with a pulse duration of less than500 fs.
 37. The machining device as claimed in claim 31, wherein theintensity modulator blocks out radiation components near the opticalaxis.
 38. The machining device as claimed in claim 31, wherein theintensity modulator effects energy distribution away from regions nearthe optical axis.
 39. A method for machining a material comprising:focusing laser radiation into or onto the material; selecting theparameters of the laser radiation and the focusing such that anon-linear absorption of the laser radiation is caused in the material;linearly polarizing the laser radiation before focusing; and varying apolarization direction across the beam cross-section.
 40. The method asclaimed in claim 39, further comprising shifting the position of thefocus of the laser radiation at least two-dimensionally.
 41. The methodas claimed in claim 39, further comprising adjusting the variation ofthe polarization direction during machining.
 42. The method as claimedin claim 41, further comprising determining and controlling a qualityparameter of machining, with the variation of the polarizationdirections being used as the manipulated variable.
 43. The method asclaimed in claim 39, further comprising focusing the laser radiation inthe vicinity of the surface of the material to be machined, with thedistance of the focus from the surface of the material to be machinedlying approximately in the range of the Rayleigh length of theradiation.
 44. The method as claimed in claim 39, further comprisingforming cut surfaces in the material by areal sequential arrangement ofoptical breakthroughs generated by non-linear absorption, the cutsurfaces being located in the material and have a cutting lineextending, up to the surface of the material.
 45. A method for machininga material, comprising: focusing laser radiation into or onto thematerial; selecting the parameters of the laser radiation and thefocusing such that a non-linear absorption of the laser radiation iscaused in the material; modifying the intensity distribution of thelaser radiation before focusing is modified, with the radiationintensity being attenuated near the optical axis.
 46. The method asclaimed in claim 45, further comprising shifting the position of thefocus of the laser radiation at least two-dimensionally.
 47. The methodas claimed in claim 45, further comprising modifying the intensitydistribution during machining.
 48. The method as claimed in claim 47,further comprising determining and controlling a quality parameter ofmachining, with the variation of the polarization directions being usedas the manipulated variable.
 49. The method as claimed in claim 45,further comprising focusing the laser radiation in the vicinity of thesurface of the material to be machined, with the distance of the focusfrom the surface of the material to be machined lying approximately inthe range of the Rayleigh length of the radiation.
 50. The method asclaimed in claim 45, further comprising forming cut surfaces in thematerial by areal sequential arrangement of optical breakthroughsgenerated by non-linear absorption, the cut surfaces being located inthe material and have a cutting line extending, up to the surface of thematerial.